AAWSAP DIRD, Aneutronic Fusion Propulsion II, November 2010

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ION PROPULSION
Ion propulsion is a method by which the principles of electromagnetics are exploited to
accelerate an ionized gas in a controlled manner. The forces involved to accelerate the
plasma, by Newton's third law, act to propel an object such as a rocket or spacecraft
along a given path. The classification of the different plasma propulsion designs is
somewhat difficult. Some designs involve electrostatic fields only, some use magnetic
fields for ionization purposes but not for ion acceleration, and some designs use both
electric and magnetic fields for ion acceleration and thrust. In addition, some designs
emit ions from a material anode, while other designs use electric and magnetic fields to
ionize a gas by one of several methods, for example, by enhancing collisions or by
using radiofrequency (RF) waves, to create a plasma . The different classes of plasma
propulsion are described in the following subsections. The use of fusion reactions in an
ion propulsion device could significantly enhance the energy of the accelerated ions
augmenting the net thrust per watt expended. We will examine the methods of plasma
ion propulsion that may benefit from aneutronic fusion.
Ion Thrusters
Ion thrusters typically emit charged particles from an anode or cathode to create an ion
population. This population is then accelerated by an electric (and sometimes magnetic)
field to generate thrust. Examples of ion thrusters include gridded electrostatic
thrusters, Hall effect thrusters, and field-emission electric propulsion systems:
Gridded Electrostatic Thrusters. Gridded electrostatic thrusters were originally
derived from a duoplasmatron design (which uses electrons from a cathode filament to
ionize an introduced gas). Such designs then accelerate and focus ions into a beam,
using an electrostatic potential, with a force equal to the ion mass times the strength of
the electric field (Coulomb force). An external (to the plasma chamber) electron gun
expels electrons into the exhaust to neutralize the system (to keep the spacecraft from
charging up, pulling the ions [exhaust] back toward the spacecraft and lowering
efficiency dramatically). These designs are typically low thrust and low specific impulse
(Isp).
Hall Effect Thruster. Hall thrusters use a magnetic field to trap electrons (which are
used to ionize the gas) and use an electric field to accelerate ions to create thrust. The
electrons also form a virtual cathode, in place of a physical grid, which is used to
accelerate the ions. Lastly, electrons are used to neutralize the exhaust. About 30
percent of the discharge current is an electron current, which does not produce thrust.
This limits the energetic efficiency of the Hall effect thruster. This design can produce a
specific impulse of ~1,500 seconds and thrusts of several tens of mN up to ~3 N.
Field-Emission Electric Propulsion Systems. A field-emission electric propulsion
(FEEP) system uses a very strong electric field to cause metal ions to be emitted from a
metal tip. These ions are then electrostatically accelerated to provide thrust. An
electron gun neutralizes the exhaust. This is typically a very-low-thrust system.
PLASMA THRUSTER
Plasma thruster designs usually ionize a gas contained within a chamber, which is then
accelerated using electric and magnetic forces (Lorentz force). Examples of plasma
thruster designs discussed in the following subsections include magnetoplasmadynamic
and Lithium Lorentz Force Accelerator (LiLFA) thrusters, electrodeless thrusters, helicon
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